The present invention relates generally to skin covering material, referred to herein as facepaint material, and more particularly to a facepaint material especially formulated to prevent or substantially reduce the possibility of detection of the user thereof by thermal imaging devices.
While the present standard military camouflage facepaint provides protection against detection of soldiers by visible means and image intensifier-type night vision devices, there exists a need to eliminate or reduce the threat of detection of the soldier either during daylight or at night by thermal imaging devices.
Thermal imagery is a rapidly developing field of technology, and new applications for it are found almost every day. For background information on this subject, one of many texts of interest is entitled "Thermal Imaging Systems" by J. M. Lloyd, Plenum Press, New York and London, 1975. This text is incorporated herein by reference.
Thermal imagers pose a serious surveillance threat to any heat source. These devices depend for their operation on a temperature difference between the target, i.e. the heat source, and the ambient temperature. It is well-known in this art that the sensor of a thermal imager can detect any heat source whose temperature is different (usually greater) from its surroundings by about 2.degree. F. (1.1.degree. C.) or more. Thermal imagers thus "see" (produce an image of) smokestacks of fixed buildings, smokestacks of ships at sea, helicopters, airplanes landing at airports, buildings the infrared reflectivity of which may be different in different parts thereof (as illustrated in the Lloyd text referenced above), human faces, etc.
Thermal imagers are therefore a dangerous surveillance threat as modern tanks and airplanes are commonly equipped with them. One danger from the military viewpoint is that such imagers can easily detect a soldier's face or other exposed skin (at approx. 97.degree. F.), his uniform, a heated tent, an armored vehicle not fully cooled down, airplane, etc. There is no contrast in thermal images of human faces to background only if the ambient temperature is coincidentally about 97.degree. F. Their advantage over other devices is that since their operation depends on emitted, rather than reflected energy, they can "see" through obscurants, operate just as well during the day as at night, can detect a target completely hidden in a shadow, and can defeat ordinary (visible) camouflage. For the reasons advanced above, it becomes important to have a safe, inexpensive and effective means to minimize the possibility of detection of a soldier by detection of the high thermal contrast which often exists between his or her face and hands and the surrounding environment.
In order to avoid detection by a thermal imager device, it is necessary to reduce the difference in temperature (.DELTA.T) "seen" by the imager device. One practical way is to lower the emissivity of the target. Under these conditions the target appears "cooler" than is actually the case. Emissivity is an optical term referring to the efficiency of radiation. It has no units and can vary from 0 to 1.0. A hypothetical "black body" has an emissivity of 1.0, i.e., 100 percent.